Dielectric end fire antenna



Nov. 26, 1968 v LENS 3,413,643

DIELECTRIC END FIRE ANTENNA Filed Nov. 26. '1965 m: I o r0- I/ l L\ I g N .4 ,E IU\\ 115 e m m I H 9 :9 H 5 311 I J I. m

F E ED INVENTOR BERNARD L. LEWIS QW Z ATTORNEYS 3,413,643 DIELECTRIC END FIRE ANTENNA Bernard L. Lewis, Satellite Beach, Fla., assignor toRadiation Incoi'porated, Melbourne, Fla., a corporation of Florida Filed Nov. 26, 1965, Ser. No. 509,802 7 Claims. (Cl. 343-785) ABSTRACT OF THE DISCLOSURE An inflatable end fire antenna has a plurality of spaced dielectric elements of like shape sharing a common longitudinal axis corresponding to the preferential direction of radiatioii from the antenna. A container encompasses the dielectric elements and provides the sole support therefor, the container being an inflatable bag to which the dielectric elements are fastened along their respective edges at positions conforming to-the desired spacing of the elements when the bag is inflated. An antenna feed is coupled to one end of the container'to excite electromagnetic waves therein, and apertures are provided for permitting ingress and egress of gas into and from the spaces between the dielectric elements for inflation and deflation of the bag; the elements each having values of dielectric constant, dimensions and spacing, preselected to provide a value of etfective dielectric constant for the overall antenna much lower than the value of dielectric constant of each of the elements themselves. r

r l The :sent invention relates generally to antennasi and more particularly to inflatable dielectric end fire: antennas.

Generally speaking, an end fire antenna is any antenna, array of antennas, or array of arrays, having a .-central axis, and so designed and excited that the maximum intensity of radiation is directed along that axis. Gonventional end fire antennas include dielectric rod, ferrite rod or tube antennas, helical antennas, Beverage antennas, V-antennas, and such arrays as the rhombic, the logperiodic and the Yagi.

Inflatable antennas per se are, of course, well known and have been constructed in a variety of shapes and sizes including element configurationsdesired to provide a particular function. In general, inflatable antennas-are desirable in many applications for their attrib'ifites of compact. storage (in the collapsed state), ease of transportatiqrijfand rapid formation into an operative structure when conditions so require. The present invention contemplates inflatable structure for just such advantages and coiisequently the antenna may readily be utilized in both ground and airborne applications, although it should be emphasized that, as in the case of all such inflatable devices, this is merely an ancillary feature and not an essential requirement in the basic principles underlying the invention nor in the practice of the invention. The primary reason for the preferred inflatable character of antennas according to the present invention will become apparent in the ensuing description.

I have found that the gain of conventional end fire antennas, such as the dielectric rod antenna, may be substantially increased by the use of very low effective dielectric constants (e) that will permit rod diameters (D) that are large relative to the wavelength (A) of the electromagnetic waves to be radiated. Typically, dielectric end fire antennas have dielectric element diameters on the order of one half wave-length (M2) of the signals of interest. In accordance with the present invention, on the other hand, the low dielectric constant element (e.g., rod or disc) diameters may be several times the wavenited States Pate length to be radiated. Several advantages are obtained by the use of such structure, among which are: the effective area of the antenna is much larger than its physical crosssectional area and thus the antenna may be arrayed with suflicient electrical overlap to eliminate grating "lobes without introducing mechanical interference, ,rnaking it an ideal array elementr the antenna has a significantly higher gain than end-fire antennas of conventional type of which I am aware; the antenna has a much larger effective area than conventional dielectric rod antennas; by virtue of the use of low dielectric constant elements the antenna is extremelylight in weight and is characterized by very low dielectric loss.

It is accordingly a primary object of the present invention to provide improved end fire antennas.

It is a more specific object of the invention to provide dielectric end fire antennas having increased gains over those attainable with conventional end fire antennas.

Another object of theflpresent invention is to provide an inflatable dielectric end fire antenna characterized by high gain, large effective area, light weight, low dielectric loss and ease of transportation in the low volume deflated state.

The above and still further objects, features and attendant advantages of the present invention will become apparent from a consideration of the followirig detailed description of preferred embodiments thereof, especially when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a side elevational view, in cross-section, of one embodiment of the invention;

FIGURE 2 is an end view, partially in section, of the antenna of FIGURE 1;;

FIGURE 3 is a side elevational view, in cross-section, of another embodimentof the invention; and? FIGURE 4 is an end view, partially in section, of the antenna of FIGURE 3.

Referring now to the drawings, and more particularly to the embodiment shown in FIGURES 1 and.2, the invention may be implemented in the form of a plurality of spaced dielectric discs 10 supported by an inflatable, thin-walled dielectric bag', or container, 13, so that when inflated, the antenna has a relatively cylindrical shape with a longitudinal central axis of symmetry. The discs 10 may be maintained in fixed spaced relationship when container 13 is inflated, by leak-proof attachment to the inner surface of the container wall in any convenient and conventional manner, such as by use of a thin layer of high strength, high vacuum-maintaining silicone cement along the points of physical contact between disc and bag.

Bag 13 is preferably composed of a suitable plastic material capable of maintaining the necessary pressure so that it may readily be inflated to assume a substantially rigid shape, and, upon evacuation of air therefrom, will collapse to form a much lower volume mass having dimensions determined primarily by the dimensions of the discs. The discs 10 may be composed of any dielectric material consistent with requirements to be presently described.

Each of discs 10, except for the disc at one end of the structure, contains an air vent 15, such as a hole, extending through its wall to permit ingress and egress of air during periods of inflation and deflation, respectively, of the supporting bag. Preferably, the air vent are aligned along the central axis of the structure, as shown, to maintain the symmetry of the structure and to prevent any abnormal deformation of the radiation pattern in any plane containing the axis in three-dimensional space. The disc at the end opposite the closed disc end may be provided with an air valve or closable port 19 to permit insertion of an inflation tube 22 and controlled inflation of the bag from any suitable source of air under pressure (not shown), as well as to permit deflation of the bag, when desired. Alternative placements of air vents and inflation port from the locations shown in the figures under discussion are, of course, within the scope of the invention. Moreover, the inflating gas need not be air so long as its dielectric constant is comparable to that of air (which has a dielectric constant, E, of approximately 1), or is at least relatively low.

The antenna embodiment of FIGURE 1 may be fed by a source of excitation in any convenient and conventional manner, as by a lens and associated feed, a parabola and associatedffeed, a horn, or an open ended waveguide (not shown) whose electrical axis coincides with the axis of symmetry of the inflated antenna and whose aperture contains one end of the dielectric rod formed by the inflated antenna. As with all end-fire antennas, maximum radiation intensity occurs along the central axis of the configuration, but, as previously stated, the antenna gain is considerably greater than the gains heretofore achieved using conventional end-fire antennas.

The pertinent design considerations are as follows:

The spacing (S) between dielectric discs 10 is deter= mined -by the thickness and the dielectric constant of the discs. If the dielectric constant of the disc is designated e and the disc thickness t, the spacing S should be such that 10 elo e where e is the desired overall dielectric constant of the dielectric rod formed by the disc loaded inflated bag. Simple manipulation of expression (1) gives where x is the free space wavelength of the radiation of interest, but when e is close to 1, this criterion is not critical.

Discs 10 may each have any dielectric constant con sistent with the requirement that S and t have the relationship indicated by expressions (1) and (2) and are chosen to make the overall dielectric constant of the rod 4D where D is the disc diameter (very nearly equal to rod diameter). Rod dielectric constants in accordance with expression (4) are effective to exclude all but the HE mode.

The eifective area of the rod is greater than the physical area because energy is refracted into the rod through the sides and adds to that coming through the end of the rod provided that the rod length L is less than or equal to the optimum length (L given by 2 (V Mathematically, then, the relationship of rod length L to the other parameters of the rod is preferably In one physical embodiment of the antenna structure shown in FIGURES 1 and 2, constructed for operation at a frequency of kilomegacycles (kmc., or gc.), the rod was a 12 inch diameter, 10 foot long foam disc loaded plastic bag driven by a 12 inch parabola and associated feed. The disc thickness was inch, disc spacing 4% inches, dielectric constant of discs 1.037, wall thickness of bag 0.006 inch, and bag wall dielectric constant 2.5. The weight of the rod was less than one pound. The radiation patterns taken in the E and H plane were substantially cross sections of a figure of revolution with a 3 db beam width of approximately 3 degrees and a gain of 32 db.

It will be noted that conventional polystyrene or ferrite rod or tube end fire antennas have elfective areas limited to approximately 3 square wavelengths at the frequencies of interest, While antennas in accordance with the present invention have been shown under test to provide efiective areas in excess of square wavelengths.

Another embodiment of an antenna in accordance with the invention is shown in FIGURES 3 and 4. Here, the rod antenna comprises a plurality of substantially cylindrical, coextensive, concentric dielectric elements 30 in the form of inflatable dielectric bags whose walls increase the dielectric constant of space in their vicinity. The bags may be separate elements which are cemented or otherwise suitably fastened together, or the rod may comprise a single cylindrical bag having a pair of closed ends (except for input ports) with internal cylindrical concentric walls forming the required elements. Each inflatable member may be provided with a separate valve or input port 33 at one end of the rod. I

The antenna of FIGURES 3 and 4 is fed in the same manner as the previously described embodiment, and is subject to the same design considerations as were pre viously described. Here, however, the overall dielectric constant of the rod and the rod length, while still giveri by expressions (4) and (6), above, are used to determine the wall thickness, wall spacing. and wall dielectric constant of the several elements constituting the rod. Obviously, because of the interrelationships between parameters the selection of certain of the parameters will automatically limit the freedom of choice as to the others. For example, the selection of wavelength and overall dielectric constant will determine the length and diameter of the rod, and the permissible values; of the remaining parameters. It will be apparent that the wall thicknesses in this embodiment are much less than the disc thicknesses of the previously described embodiment and that the dielectric constant of the walls may therefore be much greater than that of the discs, for comparable spacing.

While I have disclosed certain preferred embodiments of my invention, it will be apparent that various changes and modifications in the specific details of construction shown and described may be resorted to without departing from the spirit and scope of the invention, as defined by the appended claims.

I claim:

1. A dielectric end fire antenna comprising a plurality of spaced dielectric elements of like shape sharing a common longitudinal axis corresponding to the preferential direction of radiation from said antenna,

a container encompassing said dielectric elements and providing the sole support therefor, said container comprising an inflatable bag, said elements fastened to said container along their respective edges at positions conforming to the desired spacing thereof when said bag is inflated,

feed means coupled to an end of said container to excite electromagnetic waves therein, said feed means having an electrical axis coinciding with said longitudinal axis and having an aperture adjacent one end of said container, and

means for permitting ingress and egress of gas into and from the spaces between said elements for respective inflation and deflation of said bag, said elements each having values of dielectric constant, dimensions and spacing, preselected to provide a value of effective dielectric constant for the overall dielectric antenna much lower than the value of dielectric constant for each of said elements.

2. The antenna according to claim 1 wherein each of said dielectric elements is a disc, the discs being aligned in spaced relationship along said longitudinal axis.

3. The antenna according to claim 2 wherein the discs are provided with vents constituting said means to permit inflation and deflation of the bag.

4. The antenna according to claim l wherein said dielectric elements comprise a plurality of coextensive, coaxial di'electric cylinders maintained in fixed space relationship with each other by said fastening along said respective edges to the end walls of said container.

5. The antenna according to claim 4 wherein each of said cylinders is inflatable and defiatable.

6. An end fire antenna comprising a dielectric rod; and feed means arranged and adapted to excite electromagnetic waves in said dielectric rod; said dielectric rod including a plurality of spaced dielectric members, each having values of dielectric constant, dimensions and'spacing', preselected to provide a value of effective dielectric constant for the overall rod much lower than the value of dielectric constantrof each of said members; wherein the effective value of dielectric constant of the rod is where X is the free space wavelength of the electromagnetic waves and D is the value of the diameter of the rod, and wherein the length of the rod along the principal axis of the antenna is wherein said dielectric members comprise a plurality of coextensive, concentric dielectric cylinders maintained in fixed spaced relationship with each other.

.7. The antenna according to claim 6 wherein each of said cylinders is inflatable and defiatable.

References Cited UNITED STATES PATENTS 2,413,085 12/1946 Tiley 343785 2,588,610 3/1952 Bodthroyd et al 343785 2,936,453 5/1960 Coleman 343915 3,202,998 8/1965 Hoffman 343785 3,278,938 11/1966 Ros'jenthal 343-880 ELI LIEBERMAN, Primary Examiner. 

